1. Field of the Invention
The present invention relates to linear beam devices having multi-stage depressed collectors. More particularly, the invention relates to a crowbar circuit for use in protecting a linear beam device from damage due to arcing between the internal electrodes by quickly removing voltages from each of the electrodes upon detection of an arcing condition.
2. Description of Related Art
Linear beam electron devices are used in sophisticated communication and radar systems that require amplification of a radio frequency (RF) or microwave electromagnetic signal. Conventional klystrons, traveling wave tubes and inductive output tubes are examples of linear beam electron devices. In a linear beam device, an electron beam originating from an electron gun having a cathode is accelerated by a voltage differential with an anode spaced from the cathode. The electron beam passes through a drift tube containing an RF interaction structure. The electron beam may be amplitude modulated by applying an RF input signal between a grid and the cathode. Alternatively, the RF interaction structure of the drift tube may further include an RF circuit used to induce a modulation on the electron beam. Either way, the modulation results in electron bunching due to electrons that have had their velocity increased gradually overtaking those that have been slowed. The accelerated electrons of the electron beam give up varying amounts of their energy to the RF electric fields of traveling or standing wave circuits of the RF interaction structure. The energy removed from the electron beam in this manner may be subsequently removed from the device in the form of an amplified RF signal.
A known technique for increasing the efficiency of a linear beam device is to collect the electrons of the electron beam after they pass through the RF interaction structure using a multi-stage depressed collector. A multi-stage depressed collector includes plural collector electrodes that are each connected to power supplies that provide DC voltages ranging between the cathode potential and the maximum potential to which the electrons of the beam were accelerated. The spent electrons of the beam have various energy levels remaining as they exit the RF interaction structure, and the electrons are collected on individual ones of the collector electrodes having DC voltage levels corresponding to their remaining energy level. This way, it is possible to recover some of the remaining energy of the spent electrons that was not given up to the RF electric fields without losing this energy from conversion into heat.
A drawback with high power linear beam devices is that they sometimes arc internally, and it then becomes necessary to quickly remove all voltages from the device in order to prevent damage to the internal electrodes. By way of example, an electric arc that delivers one Joule of energy to a copper electrode can leave a hemispherical pit slightly over one millimeter in diameter. The filter capacitors of the power supplies -connected to the electrodes of a high power linear beam device typically store energy on the order of thousands of Joules. Thus, an arc discharge from one of the filter capacitors could cause substantial damage to the internal electrodes. To avoid such catastrophic events from occurring, it is known to connect each filter capacitor to a spark-gap, or "crowbar," circuit to discharge the capacitor and thereby remove the stored energy very quickly (i.e., within a few microseconds) after an arc is sensed. A single crowbar circuit has been impractical when multi-stage depressed collectors are used with a linear beam device, since each collector electrode generally utilizes a separate power supply and an associated filter capacitor to provide the corresponding voltage, and thus a separate crowbar circuit would generally be included with each filter capacitor. This duplication of circuitry tends to substantially increase the complexity and hence the cost of equipment using such a high power linear beam device.
Accordingly, it would be very desirable to provide a linear beam device with a multi-stage depressed collector in which only a single crowbar circuit is needed to remove voltages from all of the internal electrodes of the device upon the detection of an arc.